1. Field of the Invention
This invention relates to a technique of correcting for a non-linear characteristic in a display device, and more particularly to an apparatus and method that is adapted to compensate for a non-linear characteristic in a flat display device, such as a liquid crystal panel, in which an image signal is processed in a digital system and driven with an analog signal.
2. Description of the Prior Art
Recently, a flat panel display device has been developed and spread as a picture display device. Such a flat display device requires that, although picture elements(or pixels) are driven with an analog signal like a liquid crystal panel or a plasma display panel, an image signal to be applied to each pixel should be processed in a digital system. Also, the plat display device is unable to perform an electro-light conversion in the same shape as an electrical signal like the existing cathode ray tube. In other words, the flat display device generates a light signal changing non-linearly with respect to an image signal changing linearly. For example, in the case of a liquid crystal panel, a light quantity transmitting the liquid crystal panel responds non-linearly to an image signal changing linearly. The flat display device having such a non-linear characteristic referred to as a gamma error has a disadvantage in that it has a low picture quality. In order to overcome this disadvantage, a display apparatus employing the flat display device includes circuits for compensating for a non-linear characteristic in the flat display device.
FIG. 1 shows a conventional liquid crystal display apparatus having a gamma correction function. As shown in FIG. 1, the liquid crystal display apparatus includes an analog to digital converter 110, hereinafter referred to as "A-D converter", receiving an analog image signal, and a first memory 120, a digital signal processor(DSP) 130, a second memory 140, a digital to analog converter 150, hereinafter referred to as "D-A converter", and a liquid crystal panel driver 160 that are connected, in series, to the A-D converter 110. The A-D converter 110 converts an analog image signal into an 8 bit digital image signal. The digital image signal generated from the A-D converter 110 are temporarily stored in the first memory 120. The DSP 130 converts the digital image signal stored in the first memory 120 into an image signal having such a shape that is required by a flat display device, i.e., a liquid crystal panel. In other words, a shape of the digital image signal stored in the first memory 120 is converted to meet a driving condition of the liquid crystal panel by means of the DSP 130. The second memory 140 compensates for a gamma error of the digital image signal shape-converted by means of the DSP 130, that is, a non-linear characteristic of the liquid crystal panel. More specifically, the second memory 140 generates a compensating image signal having a gray scale different from the digital image signal from the DSP 130. In order to perform such a gamma compensation of the digital image signal, a look-up table, in which compensating logical values correspond to each of logical values(i.e., 256 logical values) in a digital image signal, is stored in the second memory 140. Accordingly, the digital image signal acts as an address signal of the second memory 140. Also, a part of the logical values of the compensating image signal output from the second memory 140 corresponds commonly to at least two logical values in the logical values of the digital image signal. The compensating image signal output from the second memory 140 is converted into a shape of analog signal by means of the A-D converter 150. Then, the liquid crystal driver 160 drives a liquid crystal panel(not shown) with an output signal of the D-A converter 150.
The liquid crystal display apparatus in FIG. 1 further includes a timing controller 170 and a data controller 180 that are controlled by a system controller 190. The timing controller 170 divides operation timing of the A-D converter 110, the first memory 120, the DSP 130, the second memory 140, the D-A converter 150 and the liquid crystal panel driver 160. The data controller 180 controls a timing of a signal applied from the liquid crystal panel driver 160 to the liquid crystal panel.
In the liquid crystal display apparatus using such a look-up table, a gray scale of the compensating image signal becomes smaller than that of the digital image signal. In other words, when the digital image signal has a 8 bit, the gray scale of the compensating image signal becomes smaller than 256 steps which are a gray scale of the digital image signal having 256 steps. Due to this, a gamma compensating apparatus using the look-up table lowers a gray scale of the image signal applied to the liquid crystal panel driver 160. As a result, the number of colors that can be displayed on the flat display device is reduced.
FIG. 2 shows a liquid crystal display apparatus having a gamma correction function of analog system. As shown in FIG. 2, the liquid crystal display apparatus includes a AD converter 210 for converting an analog image signal into a digital image signal, and a memory 220, a DSP 230, a D-A converter 240, a gamma corrector 250 and a flat panel driver 260 that are connected, in series, to the A-D converter 210. The memory 220 temporarily stores the digital image signal applied from the A-D converter 210. The DSP 230 converts the digital image signal stored in the memory 220 into an image signal having such a shape that is required by a flat display device(not shown), i.e., a liquid crystal panel. In other words, a shape of the digital image signal stored in the first memory 220 is converted to meet a driving condition of the flat display device by means of the DSP 230. The shape-converted digital image signal is applied to the gamma corrector 250 after the compensating image signal was converted into a shape of analog signal by means of the D-A converter 240. The gamma corrector 250 amplifies the analog image signal by a different gain in accordance with a level of the analog image signal from the D-A converter 240, thereby compensating a non-linear characteristic of the liquid crystal panel, that is, a gamma error. Then, the flat panel driver 260 drives the flat display device with the compensated analog image signal.
The flat display apparatus in FIG. 2 further includes a timing controller 270 and a gain controller 280 that are controlled by means of a system controller 290. The timing controller 270 divides an operation timing of each of the A-D converter 210, the memory 220, the DSP 230 and the gamma corrector 250. The gain controller 280 controls an amplification factor of the gamma corrector under the system controller 290. To this end, the gain controller 290 divides voltage levels of an analog image signal into a number of level areas and has gain values given to each level area. The gamma corrector 250, in which an amplification factor is controlled by means of the gain controller 280, includes a plurality of amplifiers having a different amplification factor in response to each level area of the analog image signal or a non-linear amplifier having a variable amplification factor.
In the flat display apparatus having a gamma correction function of analog system as described above, since an analog image signal is compensated by means of the plurality of amplifiers having a different amplification factor or the non-linear amplifier, a non-linear characteristic, that is, a gamma error of the flat display device can not be compensated accurately. Also, in the flat display apparatus having a gamma correction function of analog system, it is difficult to control a gain of an amplifier, so that it causes the reduced productivity as well as a different picture quality for each product.
In view of this, there is required an apparatus and method that is suitable for correcting a non-linear characteristic of a display device in such a manner to keep the number of displayable colors as it is and to unify a picture quality for each product.